Flowmeter apparatus

ABSTRACT

Three posts mounted on the underside of a ship transmit acoustic signals to and from one another for computation of ship&#39;&#39;s speed. The posts are streamlined for traveling through the water and each includes at the lower end thereof a streamlined housing having a cavity in which is positioned a transducer assembly. The cavity is covered by a thin acoustic window along the streamlined surface. Water is introduced between the transducer assembly and the window for proper signal coupling such that a propagated acoustic signal is properly transmitted to the transducer assembly in another post. Due to the streamlining, the acoustic signals impinge at different angles upon at least two of the posts and the acoustic windows for these two different posts are of different materials and carefully chosen to insure proper signal reception.

ilnited States Patent [1 1 Meyer et al.

,Fan. 1, 1974 FLOWMETER APPARATUS [75] Inventors: Thomas I. Meyer,Severna Park;

Thomas N. Shaffer, Silver Spring; Lawrence G. Wright, Pasadena, all ofMd.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: June 6, 1972 [2l] Appl. No.: 260,227

[52] US. Cl. 73/181, 73/194 B [5]] Int. Cl l. G0lc 21/10 [58] Field ofSearch 73/l8l, 194 B;

[56] References Cited UNITED STATES PATENTS 3,222,926 12/1965 Carver73/181 2,480,646 8/1949 Grabau 73/l8l Primary ExaminerDonald O, WoodielAtt0rneyF. H. Henson et al.

[57] ABSTRACT Three posts mounted on the underside of a ship transmitacoustic signals to and from one another for computation of ship'sspeed. The posts are streamlined for traveling through the water andeach includes at the lower end thereof a streamlined housing having acavity in which is positioned a transducer assembly. The cavity iscovered by a thin acoustic window along the streamlined surface. Wateris introduced between the transducer assembly and the window for propersignal coupling such that a propagated acoustic signal is properlytransmitted to the transducer assembly in another post. Due to thestreamlining, the acoustic signals impinge at different angles upon atleast two of the posts and the acoustic windows for these two differentposts are of different materials and carefully chosen to insure propersignal reception.

1] Claims, 18 Drawing Figures BACKGROUND OF THE INVENTION The inventionin general relates to flowmeter apparatus for measuring relatively fluidvelocity, and particularly, to apparatus used in the measurement ofships speed.

DESCRIPTION OF THE PRIOR ART Apparatus for measuring ships speedincludes two streamlined housings mounted on a line parallel with thefore-aft axis of the ship and wherein acoustic energy is propagatedbetween transducer assemblies in the housings. Since acoustic energytransmitted from the forward housing to the aft housing is speeded up bythe relative velocity of the water, and acoustic energy transmitted fromthe aft housing to the forward housing is retarded by the relativevelocity, the difference in propagation times is an indication of therelative velocity (speed).

The measurement of side slip as well as forward speed is highlydesirable in many installations and this is not possible with a twoelement configuration. For such measurement, prior art apparatusutilizes three streamlined units arranged as two forward units and theone aft unit with the acoustic signal paths between the two forwardunits and the aft unit forming a certain angle.

The transducer assemblies which transmit and receive the acousticsignals are positioned within cavities in the streamlined housings andthe cavities filled with a material known as Rl-IO-C rubber, which inaddition to filling the cavity, takes on the contour of the streamlinedhousing at the water interface. Because of the streamlined shape theacoustic energy in such an arrangement does not propagate normal to thehousingwater interface. Although the rubber has the same sound velocityproperties as water, the match is only at one temperature. Accordingly,the acoustic signal becomes degraded due to the refraction at certainother temperatures.

Due to this inverse relationship of the sound velocitytemperaturebetween the rubber and water, the velocity calculation based upon thetime difference in travel ofacoustic energy is in error since the timespent in the rubber medium by the acoustic signal varies withtemperature.

The use of the rubber also tends to limit the maximum measurable speedsince at higher velocities the rubber would deform and cavitation wouldoccur causing a loss of acoustic signal.

SUMMARY OF THE INVENTION Flow meter apparatus is provided which includesa plurality of transducer stations interfacing with a fluid medium, therelative velocity of which is to be measured. Each station includes astreamlined housing having a cavity in which is positioned a transducerassembly for projecting and/or receiving acoustic energy along a signalpath. A relatively thin acoustic window covers the cavity at thehousing-water interface and a coupling medium such as water is providedbetween the transducer assembly and the acoustic window. The commonsignal path between a first and second of the transducer stations is ata first angle with respect to the acoustic window of the firsttransducer station and at a second and different angle with respect tothe acoustic window of the second transducer station. In order to avoidsignal degradation the acoustic windows of the first and secondtransducer stations are of carefully chosen and different materials.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the flowmeterapparatus mounted on the underside of a ship;

FIG. 2 illustrates the relative positioning of the ship mountedtransducers stations of FIG. 1;

FIG. 3 is a side view of the lower housing portion of one of the forwardunits illustrated in FIG. 1;

FIG. 4 is a cross-sectional view along the lines IV-IV of FIG. 3 andillustrates the prior art.

FIG. 5 is a curve illustrating the temperature versus velocitycharacteristics of water and RI-IO-C rubber;

FIGS. 6A and 6B are cross-sectional views as in FIG. 4 but illustratethe preferred embodiment of the present invention;

FIG. 7 is a cross-sectional view of a typical transducer assembly whichmay be utilized herein;

FIGS. 8 and 9 are ray diagrams illustrating the principle of reflectionand refraction of acoustic energy traveling in one medium and incidenton a surface bounding a second medium in which the velocity ofpropagation differs from that in the first;

FIGS. 10A and 10B illustrate, respectively, a desired signal to beprocessed, and a degraded undesirable signal which the present inventionobviates;

FIG. 11 is a ray diagram similar to FIG. 9 illustrating a varyingcritical angle;

FIGS. 12 and 13 are also ray diagrams to aid in an understanding of thepresent invention;

FIG. 14 is a cross-sectional view illustrating the third housing;

FIG. 15 is a cross-sectional view of the aft housing showing thepositions of two cavities therein; and

FIG. 16 is a side view of the lower streamlined housing with a portionbroken away.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although the present inventionis applicable in various fluid flow measuring systems, it will bedescribed in particular with respect to apparatus for measuring shipsspeed, an example of which is illustrated in FIG.

Mounted on the underside of ship 10 are three transducer stations 12,13, and 14, each in a form of a streamlined post to minimize waterturbulence.

At the lower portion of each post and forming a continuation thereof,there is provided a respective streamlined housing 15, 16, and 17, whichcontain acoustic projection and reception transducer assemblies. Thestreamlined shapes and respective positioning of these housings areillustrated in FIG. 2.

The port and starboard streamlined housings are represented respectivelyby the numerals 15 and 16, and the aft streamlined housing isrepresented by the numeral 17. S represents the path of acoustic signaltransmission between transducer stations 12 and 14, and moreparticularly, between transducer assemblies positioned in housings l5and 17. Similarly, S is the path starboard signal transmission betweenthe aft housing 17 and the stargoard housing 16. In actuality, signaltransmission between the housings is in the form of compressional wavesthrough thefluid medium, with the waves having a certain pressureintensity relative to the path of transmission in accordance with acertain beam pattern. This is well known to those skilled in the art,and for ease of explanation, the waves will be represented herein bystraight lines or rays.

The housings are oriented such that the paths S and S form an angle a,preferably 90, and the forward direction of the ship is represented bythe arrow F which is parallel to the fore-aft axis of the ship.

Although various acoustic flow measuring schemes may be utilized, it ispreferable that a single path arrangement between transducer stations beutilized with the transducer assemblies acting both as a transmitter anda receiver of projected acoustic energy. Signal processing apparatus 20provides the necessary energizing signals to transducer assemblies inhousings l5 and 17 for simultaneously and repetitively projectingacoustic pulses toward one another and is thereafter operable to computethe difference in time arrivals of transmitted acoustic pulses betweenthese units. The signal processing apparatus is also operable tosimultaneously supply energizing signals to transducer assemblies inhousings l7 and 16 for projection of acoustic energy along path S andthereafter process the received signals such that ship's forward andsideway velocity components may be obtained.

FIG. 3 illustrates a side view ofa streamlined housing such asconnectable with the lower end of transducer station 12. Acoustic energyis transmitted and received through an aperture which in the prior artapparatus had been filled with a rubber material having acoustictransmission properties similar to seawater. FIG. 4 is a view along lineIV-IV of FIG. 3 and illustrates a typical prior art arrangement.

A hole or cavity 28 was formed in the housing 15 and a transducerassembly 30 was inserted therein. The remainder of the cavity betweenthe transducer assembly 30 and the outside surface of the housing 15 wassealed with the aforementioned rubber 32 and faired to the housing 15 atthe surface.

Under ideal operating conditions, such an arrangement providedsatisfactory results. A difficulty arises, however, since in reality theacoustic transmission properties of the rubber 32, as the temperaturevaries, are inversely related to the acoustic transmission properties ofwater and consequently, the velocity of sound in both mediums was thesame only at a particular temperature. This relationship is illustratedin the curves of FIG. 5, wherein the vertical axis representstemperature and the horizontal axis represents velocity. From FIG. 5 itcan be seen that as temperature increases, the velocity of sound inwater also increases, whereas in the rubber, just the opposite is trueand consequently there is a match at only one temperature T where thevelocity of sound is the same, V in both mediums.

Due to this phenomena, and with reference again to F IG. 4, refractionof the acoustic signal would occur at the interface 35 which wouldprogressively get worse and cause loss of acoustic signal as largerdeviations from T occurred.

Additionally, the velocity calculation was in error at all buttemperature T since the transit time in the rubber 32 would vary astemperature varied.

Although the rubber 32 was faired to the housing 15, such fairing wasextremely difficult to obtain and at higher velocities the rubberdeformed due to a pressure gradient across its face thereby resulting incavitation at higher speeds and a consequent loss of acoustic signal.

The above problems were totally eliminated by the present invention, apreferred embodiment of which is illustrated in FIGS. 6A and 6B whichare views, as in FIG. 4, of port housing 15 and aft housing 17,respectively.

Referring to FIG. 6A, a cavity 39 may be drilled or formed in housing 15and a transducer assembly 30 placed therein. One type of transducerassembly which may be utilized is illustrated in cross-sectional view inFIG. 7 to which reference is now made.

The assembly 30 includes a transducer active element 42 such as bariumtitanate, with a suitable backing means such as an absorber 44 supportedor backed by a pressure plate 46 carrying pins 49 for making electricalconnection with the active element 42 in a well known manner. Theassembly is then covered by a thin layer of elastomerical material 52.Since the covering 52 is so thin, any acoustic transmit time variationthrough it as the temperature changes is insignificant.

Referring back to FIG. 6A, a relatively thin acoustic window 55 coversthe cavity 39 and is bonded to the housing 15. The transducer assembly30 is spaced from the thin acoustic window 55 for transmission andreception of acoustic energy along the signal path S, also illustratedin FIG. 2. A coupling medium 57 is interposed between the transducerassembly 30 and the acoustic window 55 and has acoustic transmissionproperties similar to the medium in which the apparatus is to operate,in the present case, seawater. In the preferred embodiment, the couplingmedium 57 is actually water.

In those operational situations where water might tend to evaporate,thus requiring constant replenishment, it is preferred that suchcoupling medium 57 take the form of water held in a colloidalsuspension. One example of this is a gel which is a colloid in which thedisperse phase has combined with the continuous phase to produce ajelly-like product. A medium which has been successfully used in thepresent invention is agaragar, which prior to mixing with the water, isa dried hydrophilic, colloidal substance extracted from various marinespecies of Gelidium and Algae.

The housing 17 of FIG. 6B is similar in that it includes a cavity 60 inwhich is positioned a transducer assembly 30 spaced from a thin acousticwindow 63 with the space between them filled with a coupling medium 57such as the agar-agar described. Due to the orientation of the housings,as illustrated in FIG. 2, and due to the streamlined curvilinear surfaceof the housings, the signal path S lies at an angle 4:, with respect tothe normal N of acoustic window 55 of the port housing 15, and lies at adifferent angle (1) with respect to the normal N of acoustic window 63of the aft housing 17 Although seemingly insignificant, the differencebetween these two angles d), and 4: could conceivably result inextraneous and unwanted acoustic signals which would tend to causeimproper operation and reduce accuracy. This problem is best explainedwith initial reference to FIG. 8 illustrating the basic theory ofreflection and refraction at plane surfaces.

In FIG. 8 ray 66, representing a longitudinal acoustic wave, travelsthrough a first medium in which the velocity of sound is V. The wavestrikes a second medium in which the velocity of sound is V, at an angled: measured from the normal N. The longitudinal wave is reflected at theinterface, the reflected wave being represented by ray 67, and is alsotransmitted into the second medium as a longitudinal wave L lying at anangle o with respect to the normal N. The relationship between theangles and the velocity of sound is given as:

sine tb/sinetp' V/V FIG. 8 is drawn for the situation where V is greaterthan V. If the second medium is a solid, the incident wave, in additionto producing a longitudinal wave L, will also produce a shear wave S.There is a critical angle 15, for the incident longitudinal wave forwhich the refracted wave emerges tangent to the surface. That is, (1) is90 and sine :15 is 1. Equation 1 therefore becomes:

sine 4), V/ V This situation is illustrated in FIG. 9 where the secondmedium is illustrated as a thin window 68 having water on either side ofit. At the critical angle a shear wave is propagated in the window 68which when striking the lower interface is converted back to alongitudinal wave L,,.

FIG. 9, however, represents the idealized situation. In actuality, asapplied to the apparatus described, the thin window 68 has a slightcurvature following the streamlining of its housing. Additionally thewave front of impinging acoustic energy is not exactly planar but issomewhat spherical. (fi therefore is a nominal critical angle and theabove factors result in a longitudinal wave being propagated in thewindow which emerges on the other side of the window as a longitudinalwave L shown dotted.

It may be stated that in general any incident wave striking the window68 within the angular region L will produce a predominantly longitudinalmode wave in the window 68 and the wave L on the other side of thewindow 68 will predominate. If the incident ray strikes the interfacewithin the angular region S, the shear wave, and consequently thelongitudinal wave L, will predominate.

If acoustic window 68 is part of an arrangement such as illustrated inFIG. 6A or 6B, the effect of the two waves L and L, would be to causesignal degradation. By way of example, this is illustrated in FIGS. Aand 108 in which FIG. 10A illustrates a typical signal provided by atransducer assembly upon impingement of a high frequency acoustic pulse.In order to measure time difference of arrival of oppositely transmittedacoustic pulses, it is necessary to start the time measurement processwhen the first half cycle 70 is received and detected. This first halfcycle is sufficiently distinguishable over the noise 72 and can easilybe detected. Since the travel times of the shear and longitudinal wavesare different, if two different signals such as L and L, of FIG. 9 arereceived by the transducer assembly at different times an interferenceresults wherein the first half cycle 70, as illustrated in FIG. 10B,cannot be adequately detected above the noise 72 and consequently the'thresholding apparatus for detecting this half cycle may not provide theproper output signal until several half cycles later resulting inimpaired accuracy of the overall system.

If the apparatus could be arranged such that the incoming (or outgoing)acoustic energy could impinge upon the respective acoustic windows onlyin the region L in FIG. 9, or only in the region S, satisfactoryoperation could be obtained. It would be preferred, however, that theincident signal strike the window at an angle at least 5 removed fromthe critical angle (b A problem arises, however, in that the criticalangle 1b,, is dependent upon the velocity V and for the typicalenvironment illustrated, the velocity V varies as the temperaturevaries.

FIG. 11 illustrates the critical angle 4%, but as the water temperaturechanges throughout its changeable range, the critical angle also changesthroughout a range from (1),. to (15,". Keeping these principles inmind, and with reference again to FIGS. 6A and 68, it is necessary thatthe signal path S strike the acoustic windows 55 and 63 at an angle atleast 5 removed from the critical angle for all variations of thecritical angle due to temperature changes. In order to accomplish thisresult, and in view of the fact that angles (b, and d); are different,the present invention provides for acoustic windows 55 and 63 being madeof different materials. By way of example, with the orientation as in FIG. 2, angle (I), would be equal to approximately 505 and angle would beequal to approximately 35.5". As suming a velocity variation in water offrom 1,400 meters per second to l,580 meters per second, acoustic window55 could be an acrylic such as a polymerized methyl methacrylate, oneexample being sold under the trade name Plexiglas.

The aft acoustic window 63 could then be made of a polyester such as anacrylonitrile butadienestyrene, one example being sold under the tradename of ABS.

The longitudinal acoustic velocity in Plexiglas is approximately 2,647meters per second Substituting in equation (2), the critical angle forFIG. 6A would vary between approximately 31.9 and 366 due to thevelocity variation in the water. This variation in critical angle isshown as the shaded area in FIG. 12.

With the aft acoustic window 63 being ABS plastic, wherein the acousticvelocity is approximately 2,100 meters per second, the critical anglewould vary between approximately 41 .8 and 49, such variation being theshaded zone 77 in FIG. 13.

FIGS. 12 and 13 have been oriented similar to FIGS. 9 and 11 but inactuality represent the situations for the arrangement of FIGS. 6A and6B. In FIG. 12, the acoustic signal path S lies at an angle 4:, of 505which is more than 5 removed from the critical angle zone 75. Forpurposes of comparison, if the aft acoustic window 63 were also made ofPlexiglas, the acoustic signal path (shown dotted) lying at an angle of35.5 would fall right in the critical angle zone 75, and system accuracywould be severely degraded as previously discussed.

In FIG. 13 illustrating the situation for the aft housing, the signalpath S lying at an angle of 355 is more than 5 removed from the criticalangle zone 77. If the port acoustic window 55 were also made of ABSplastic, the signal path S (shown clotted) lying at an angle of 505would not be at least 5 removed from the critical angle zone 77 anddegraded operation would occur.

Thus, proper operation is attained with the use of different materialswhose respective speed of sound is chosen such that the signal pathangle is at least approximately removed from the critical angledetermined by the longitudinal velocity of propagation, for allvariations of the critical angle due to temperature changes. Therelatively thin plastic acoustic windows provide a hard surface which iseasily contoured to the proper shape of the respective housings andresult in a smooth streamlined surface which does not deform atincreased ship velocities.

As illustrated in FIG. 14, the transducer housing unit 16 is similar tothe arrangement illustrated in FIG. 6A in that the housings have thesame contour and streamlined shape, the angles qb are of the samemagnitude, and the acoustic windows 55 are of identical material. Eachof the forward housings, and 16 include one cavity and transducerassembly whereas the aft housing 17 would include two transducerassemblies, one for communication with the transducer assembly inhousing 15 and the other for communication with the transducer assemblyin housing 16 as illustrated in FIG. 15. This second transducer housingwould be located within a cavity 95 in the housing 17, and wouldtransmit through an acoustic window identical to acoustic window 63 withagar-agar disposed between the transducer assembly and the acousticwindow. The signal path would be at the same angle 41 with respect tothe acoustic window except emanating from the right side of the housing17. Since the width of the housing 17 is relatively narrow, suchadditional cavity 95 is drilled at a lower (or higher) position in thehousing 17.

FIG. I6 is a view of a typical housing such as 15 with a portion cutaway to illustrate the method of introducing the coupling medium 57. Inthe preferred embodiment a distilled water solution containing 0.25percent agar-agar by weight is first boiled and then poured into cavity39, while in its liquid state, through fill column 80. After thesolution gels, a thin layer of an elastomeric such as rubber 82, isplaced on top of the gel to seal the cavity. A countersunk portion 84 ofthe fill hole is then sealed with an aluminum disk 87 and a second layerof rubber 89, preferably an RTV silicone rubber. This arrangementresults in a short column of air 92 which together with the thin sealinglayer of rubber 82 permits thermal expansion of the solution withoutsignificantly increasing the internal pressure. The agaragar gel, unlikepure water, is capable of supporting the rubber and its use reduces thepossibility of air entrapment. The agar-agar gel is not identical to theliquid medium (sea-water) in which the apparatus operates, however,tests have indicated that there is no appreciable difference in theacoustic property of the agar-agar solution as compared to seawater.

We claim:

1. Flowmeter apparatus comprising:

a. a plurality of transducer stations for interfacing with a fluidmedium;

b. each said transducer station including a housing having a cavitytherein, a transducer assembly and a relatively thin acoustic windowcovering said cavy;

c. said transducer assembly being positioned within said cavity andspaced from said acoustic window for projection and/or reception ofacoustic energy therethrough, along a signal path to or from thetransducer assembly of another said transducer station;

d. a coupling medium disposed between said transducer assembly and saidacoustic window and having acoustic transmission properties similar tothe medium in which the apparatus is to operate;

e. a signal path between a first and second of said transducer stationswhen in operation being at a first angle with respect to the acousticwindow of said first transducer station and being at a second anddifferent angle with respect to the acoustic window of said secondtransducer station;

f. the acoustic windows of said first and second transducer stationsbeing of different materials.

2. Flowmeter apparatus for measuring ships speed comprising:

a. at least 3 posts for mounting on said ship for communication with thewater;

b. each said post having a respective streamlined housing connectedthereto;

c. each said housing including a cavity, a transducer assembly and arelatively thin acoustic window covering said cavity;

d. said transducer assembly being positioned within said cavity andspaced from said acoustic window for projection and for reception ofacoustic energy therethrough along a signal path;

e. a third of said housings including an additional cavity, acousticwindow and transducer assembly;

f. an acoustic coupling medium disposed between each said transducerassembly and its respective acoustic window;

g. said housings being arranged that the transducer assemblies in thefirst and third housings are in signal communication along a firstsignal path and in the second and third housings are in signalcommunication along a second signal path.

3. Apparatus according to claim 2 wherein:

a. said acoustic windows of said first and second housings being of thesame material and being of a different material from the acoustic windowof said third housing.

4. Apparatus according to claim 2 wherein:

a. said first and second signal paths are at an angle of with respect toone another.

5. Apparatus according to claim 2 wherein:

a. each said post forms a streamline continuation of its respectivestreamlined housing.

6. Apparatus according to claim 2 wherein:

a. each said acoustic window forms a streamlined continuation in thesurface of its respective housing.

7. Apparatus according to claim 6 wherein:

a. each said acoustic window is bonded to its respective housing.

8. Apparatus according to claim 2 which includes:

a. signal processing equipment for providing and receiving signals toand from said transducer assemblies and for processing said signals forcomputation of ships speed.

9. Apparatus according to claim 3 wherein:

a. the longitudinal acoustic velocity in the acoustic windows of saidfirst and second housings is greater than the longitudinal acousticvelocity in the acoustic windows of said third housing.

10. Apparatus according to claim 2 wherein:

a. said acoustic coupling medium is a gel.

c. said acoustic windows being of different materials whose respectivespeed of sound is chosen that the signal path angle is at leastapproximately 5 removed from the critical angle determined by thelongitudinal velocity of propagation, for all variations of the criticalangle due to temperature changes.

1. Flowmeter apparatus comprising: a. a plurality of transducer stationsfor interfacing with a fluid medium; b. each said transducer stationincluding a housing having a cavity therein, a transducer assembly and arelatively thin acoustic window covering said cavity; c. said transducerassembly being positioned within said cavity and spaced from saidacoustic window for projection and/or reception of acoustic energytherethrough, along a signal path to or from the transducer assembly ofanother said transducer station; d. a coupling medium disposed betweensaid transducer assembly and said acoustic window and having acoustictransmission properties similar to the medium in which the apparatus isto operate; e. a signal path between a first and second of saidtransducer stations when in operation being at a first angle withrespect to the acoustic window of said first transducer station andbeing at a second and different angle with respect to the acousticwindow of said second transducer station; f. the acoustic windows ofsaid first and second transducer stations being of different materials.2. Flowmeter apparatus for measuring ship''s speed comprising: a. atleast 3 posts for mounting on said ship for communication with thewater; b. each said post having a respective streamlined housingconnected thereto; c. each said housing including a cavity, a transducerassembly and a relatively thin acoustic window covering said cavity; d.said transducer assembly being positioned within said cavity and spacedfrom said acoustic window for projection and for reception of acousticenergy therethrough along a signal path; e. a third of said housingsincluding an additional cavity, acoustic window and transducer assembly;f. an acoustic coupling medium disposed between each said transducerassembly and its respective acoustic window; g. said housings beingarranged that the transducer assemblies in the first and third housingsare in signal communication along a first signal path and in the secondand third housings are in signal communication along a second signalpath.
 3. Apparatus according to claim 2 wherein: a. said acousticwindows of said first and second housings being of the same material andbeing of a different material from the acoustic window of said thirdhousing.
 4. Apparatus according to claim 2 wherein: a. said first andsecond signal paths are at an angle of 90* with respect to one another.5. Apparatus according to claim 2 wherein: a. each said post forms astreamline continuation of its respective streamlined housing. 6.Apparatus according to claim 2 wherein: a. each said acoustic windowforms a streamlined continuation in the surface of its respectivehousing.
 7. Apparatus according to claim 6 wherein: a. each saidacoustic window is bonded to its respective housing.
 8. Apparatusaccording to claim 2 which includes: a. signal processing equipment forproviding and receiving signals to and from said transducer assembliesand for processing said signals for computation of ship''s speed. 9.Apparatus according to claim 3 wherein: a. the longitudinal acousticvelocity in the acoustic windows of said first and second housings isgreater than The longitudinal acoustic velocity in the acoustic windowsof said third housing.
 10. Apparatus according to claim 2 wherein: a.said acoustic coupling medium is a gel.
 11. Flowmeter apparatuscomprising: a. at least two transducer stations each including anacoustic window and transducer means for projection and/or reception ofacoustic energy through said window along a signal path; b. said signalpath lying at a first angle with respect to one of said acoustic windowsand at a second and different angle with respect to the other acousticwindow; c. said acoustic windows being of different materials whoserespective speed of sound is chosen that the signal path angle is atleast approximately 5* removed from the critical angle determined by thelongitudinal velocity of propagation, for all variations of the criticalangle due to temperature changes.